Films fabricated from linear low density polyethylene (LLDPE) typically have higher coefficients of friction (COF) and exhibit higher blocking force than many resin converters and consumers find acceptable. Blocking force (i.e., unwanted adhesion) is usually measured according to ASTM D 3354, where the film-to-film adhesion is expressed as grams of blocking load which causes two layers of film to separate. Resin converters also often require low COF for their fabrication processes, while consumers desire products exhibiting low block because the sacks, bags, etc. are easier to open. The blocking property of film is generally considered to be unsatisfactory when the blocking force is about 20 grams or more. Resin manufacturers can incorporate additives into their LLDPE polymers to decrease the COF and the block, but this increases their production costs and the additives can adversely affect mechanical film properties such as tensile strength, dart impact, tear, etc. of the film. In addition, the additives can exude to the surface of the film (a phenomena referred to as "bloom") and thereby cause more problems, such as odor and poor adhesion to inks used to print on the surface of the finished product.
In an attempt to improve the physical properties of the film, some converters are making film from high density polyethylene, which, when converted into film, achieves good blocking force without the use of additives, thereby retaining more of the other physical properties. For example, it was reported in Plastics Engineering (August 1991) that high molecular weight high density polyethylene (HMW-HDPE) is expected to displace LLDPE in trash bag and liner applications, due to the ability of the HWM-HDPE to be made at thinner gauges, while maintaining high film strength and good blocking force, without having to use additives. Films made from HDPE, however, have other problems associated with them, including poorer dart impact strength, high splittiness, higher film stiffness and difficult heat sealing.
In spite of the potential drawbacks of making film from LLDPE, commercial producers have introduced additives into their LLDPE resins in order to reduce block and the COF in films of LLDPE. For instance, silicon dioxide, SiO.sub.2, is used as an "antiblock" agent to reduce blocking force and erucamide as a "slip" agent to reduce COF. This is a standard-practice in the industry, although various other compounds, blends, etc., may also be used.
Many U.S. Patents disclose various methods of improving slip and block properties of film made from polyethylene. For example, U.S. Pat. No. 2,770,608 claims blends of polyethylene with fatty acid amides as a means of improving both slip and block.
U.S. Pat. No. 3,104,232 discloses blends of polyethylene with N-allyl fatty acid amides as a means of improving slip.
U.S. Pat. No. 3,108,981 discloses the use of N-substituted amides of fatty acids as a means of reducing slip.
U.S. Pat. No. 3,170,889 discloses the use of mono and di-N-alkyl-substituted dicarboxylic acid diamides as a means of reducing both slip and block.
U.S. Pat. No. 3,205,190 discloses blends of elaidamide with polyethylene as a means of reducing both block and slip.
U.S. Pat. No. 3,396,137 discloses the use of N-(2-hydroxyethyl)-cis-13-docosenamide to reduce COF.
U.S. Pat. No. 3,489,824 discloses the copolymerization of various substituted acrylamides as a means to reduce both COF and block.
U.S. Pat. No. 3,647,738 discloses the use of fatty acid amides, N-substituted fatty acids, and hydroxylated derivatives of both to be used to reduce both block and slip.
U.S. Pat. No. 4,136,072 discloses the use of plasticizers blended into polyethylene to reduce block. There appears to be intense interest in reducing block and COF in LLDPE blown films and in these cases, the means to reduce the block and slip is by using an additive.
The current invention avoids all of the problems associated with the introduction of additives into LLDPE, including the additional cost for the additive itself and the necessary equipment to put it into the resin.